1. Field of the Invention
The present invention relates to a developing device for developing a latent image on a latent image carrier with a thin developer layer formed on a developer carrier by a layer forming member under application of an AC-superimposed bias voltage to the developer carrier, the AC-superimposed bias voltage being formed by superimposing an alternating current on a DC bias voltage.
2. Discussion of Related Art
Conventionally, developing devices are arranged to apply a DC bias voltage to a developer carrier to form a thin layer of a developer on the developer carrier by a layer forming member and to allow the developer to move and adhere to an image area on a latent image carrier. In one type of conventional developing devices, an alternating current is superimposed on the DC bias voltage in order to vibrate the developer and to thereby facilitate the movement of the developer from the developer carrier to the image area on the latent image carrier (for example, see Japanese Patent Application Post-Exam Publication No. Sho 58-32375). This type of developing devices adopts the non-contact jumping development in which a gap is provided between the developer carrier and the latent image carrier, and the developer is caused to fly from the developer carrier to the image area on the latent image carrier. The amplitude (Vmaxxe2x88x92Vmin) of the alternating current is, as shown in FIG. 1, set to a value exceeding the width between the non-image area potential V0 and image area potential Von of the latent image carrier. The reason for this is that the threshold value of the bias voltage sufficient to allow the developer to adhere to the image area on the latent image carrier is higher than the electric potential at the non-image area, and conversely, the threshold value of the bias voltage sufficient to separate the developer adhering to the non-image area is lower than the electric potential at the image area on the latent image carrier.
Meanwhile, a developing device has been proposed in which an AC-superimposed bias voltage is applied to a developing member provided in opposing relation to a latent image retaining member, and a constant-voltage bias is applied to a developer conveying member for conveying a developer to the developing member, thereby forming an electric potential gradient between the two members to supply the developer (for example, see Japanese Patent Application Post-Exam Publication No. Hei 3-21906). There has also been proposed another type of developing device in which a constant-voltage bias is applied to a developer carrier provided in opposing relation to a latent image carrier, and a constant-current bias is applied to a toner supply member placed in contact with the developer carrier, thereby allowing a constant electric current to flow to the toner supply member from the developer carrier (for example, see Japanese Patent Application Unexamined Publication (KOKAI) Nos. Hei 9-106172 and Hei 10-104936).
The conventional developing devices suffer, however, from some problems as stated below. The developer adhering to the non-image area on the latent image carrier cannot sufficiently be separated. Accordingly, the developer is likely to adhere to the non-image area, causing fogging. Further, blur may occur in a halftone image owing to disconnection, thickening or scattering of thin lines of the image. This causes image quality degradation. In color image formation, in particular, if there occurs such fogging or blur due to disconnection, thickening or scattering of thin lines of the image, it becomes impossible to provide satisfactory colors in halftone because a color image is outputted in the form of a combination of various color materials superimposed on one another. To minimize these problems, high-precision control is required for the gap between the developer carrier and the latent image carrier.
Further, in a case where an AC-superimposed bias voltage is applied as a developing bias voltage, if the bias applied to the toner supply member is subjected to constant-voltage control, the electric potential cannot follow the alternating current of the developing bias voltage but acts as a constant potential at all times. Accordingly, the bias may become an inverted electric potential that acts in a direction in which the developer separates from the developer carrier toward the toner supply member. Alternatively, it may become impossible to provide the desired potential difference even if the bias does not act in the separating direction. Therefore, stable supply of toner cannot be ensured. As a result, undesired brush marks occur on the developer carrier, and toner deterioration occurs with time. In addition, the resistance between the developer carrier and the toner supply member changes with time, causing a delay in the supply of toner. This makes it impossible to obtain favorable images. If the supply voltage is increased, the required toner supply can be ensured, but the amount of toner conveyed becomes excessively large. Consequently, image defects such as stripes due to positive charge occur in the developed image. Further, fogging occurs in the developed image.
In contact development type developing devices, an electrically charged one-component developer is conveyed from a developer carrier to a latent image carrier placed in contact with the developer carrier to develop an electrostatic latent image on the latent image carrier with the one-component developer. In this case, a metal roller made of aluminum or iron-base material is used as the developer carrier. In particular, an aluminum roller is frequently used because it is easy to form by machining and less costly.
Incidentally, the developer carrier used in the developing device is demanded to have the functions of {circle around (1)} conveying the developer, {circle around (2)} electrically charging the developer, and {circle around (3)} preventing discharge of the developing bias voltage.
To improve the developer conveying performance and the developer chargeability, a carrier roll (i.e. developer carrier) has heretofore been proposed in Japanese Patent Application Post-Exam Publication No. Hei 6-46331 in which the surface of a metal roller is sandblasted to form a dimpled surface, which is then subjected to metal plating treatment, e.g. nickel plating. With the carrier roll disclosed in the post-exam publication, the dimpled surface formed on the carrier roll allows the developer conveying capability to be enhanced mechanically. Thus, the developer conveying performance is improved. Moreover, the dimpled surface allows an increase in the area of contact with the developer and hence permits an improvement in the developer chargeability. Further, the wear resistance of the dimpled surface of the metal roller is improved by subjecting the dimpled surface to metal plating treatment.
To prevent discharge of the developing bias voltage, a developer carrier having a resistivity set to a predetermined value has heretofore been proposed. For example, Japanese Patent Application Post-Exam Publication No. Hei 2-26226 proposes a non-magnetic one-component toner carrier (i.e. developer carrier) comprising a cylindrical rigid member formed of a resin material with an electrically conductive powder dispersed therein and having a resistivity in the range of 104 to 1012 xcexa9cm. The inner surface of the cylindrical rigid member is formed with an electrically conductive film or coated with an electrically conductive paint having a resistivity of not more than 107 xcexa9cm. Japanese Gazette Containing the Patent No. 2705090 proposes a non-magnetic one-component toner carrier (i.e. developer carrier) having a semiconductive layer with a thickness of 100 to 1000 micrometers formed on the surface thereof by using a ceramic material, e.g. alumina, with a resistivity of 104 to 1012 xcexa9cm. With the non-magnetic one-component toner carriers disclosed in these official gazettes, because at least the surface thereof has a predetermined resistivity, the discharge of the developing bias voltage can be effectively prevented. Thus, the occurrence of image defects can be prevented.
Meanwhile, as disclosed in Japanese Patent Application Post-Exam Publication No. Hei 2-26226 and Japanese Gazette Containing the Patent No. 2705090, the conventional developing devices use a developing bias voltage formed by superimposing an AC voltage on a DC voltage to prevent undesired toner adhesion to the non-image area on the latent image carrier (i.e. fogging) and, at the same time, to provide a moderate edge effect and to improve gradation characteristics.
In the carrier roll disclosed in Japanese Patent Application Post-Exam Publication No. Hei 6-46331, however, the sandblasted dimpled surface is subjected to metal plating treatment. The plating treatment causes the plating material to be overlaid on the dimpled surface. Consequently, the clear dimple configuration formed by the sandblasting treatment is deformed by the plating material. That is, projections on the dimpled surface, i.e. edges at the boundaries between adjacent recesses, are deformed. Consequently, the dimples become unclear. Therefore, even if clear dimples are formed by the sandblasting treatment to improve the dimpled surface in wear resistance, the dimples are made unclear by the metal plating. Accordingly, it becomes impossible to sufficiently and surely obtain the effects of the dimples formed on the developer carrier surface to improve the toner conveying performance and the toner chargeability.
The developer carrier disclosed in Japanese Patent Application Post-Exam Publication No. Hei 2-26226, which is formed of a resin material having an electrically conductive powder dispersed therein, involves the problem that because an electrically conductive powder is dispersed in the resin material, the developer carrier is likely to be affected by the dispersed condition of the powder. Therefore, it is difficult for the carrier surface to have a uniform resistance. Accordingly, density unevenness is likely to occur in the developed image.
The toner carrier disclosed in Japanese Gazette Containing the Patent No. 2705090, which is formed with a semiconductive layer of a ceramic material having a thickness of 100 to 1000 micrometers, suffers from the problem that the manufacture thereof is complicated and the costs are unfavorably high because the semiconductive layer is formed by spraying the base material of the toner carrier with ceramic particles melted by arc discharge.
Moreover, it is desired that the above-described three functions {circle around (1)} to {circle around (3)} be imparted to the developer carrier even more surely. Therefore, it is conceivable to impart the three functions to the developer carrier by combining together the technical matters disclosed in the above-described official gazettes. However, the following problems arise when the techniques disclosed in the official gazettes are combined together to impart the three functions to the developer carrier.
That is, in combination of the techniques disclosed in Japanese Patent Application Post-Exam Publication Nos. Hei 6-46331 and Hei 2-26226, the carrier formed of a resin material having an electrically conductive powder dispersed therein as set forth in Japanese Patent Application Post-Exam Publication No. Hei 2-26226 is not a metallic carrier; therefore, it is difficult to form dimples by sandblasting treatment and to perform a treatment for improving the wear resistance of the dimpled surface as stated in Japanese Patent Application Post-Exam Publication No. Hei 6-46331. Accordingly, it is impossible to combine together the techniques disclosed in Japanese Patent Application Post-Exam Publication Nos. Hei 6-46331 and Hei 2-26226. It is extremely difficult to impart the above-described three functions to the developer carrier even more surely.
In combination of the techniques disclosed in Japanese Patent Application Post-Exam Publication No. Hei 6-46331 and Japanese Gazette Containing the Patent No. 2705090, a semiconductive layer of a ceramic material melted by arc discharge as stated in Japanese Gazette Containing the Patent No. 2705090 is formed on a dimpled surface formed as set forth in Japanese Patent Application Post-Exam Publication No. Hei 6-46331. Accordingly, the edges at the boundaries between the adjacent recesses are deformed and hence the dimples become unclear as in the case of subjecting the dimpled surface to metal plating as stated in Japanese Patent Application Post-Exam Publication No. Hei 6-46331. For this reason, it is impossible to combine together the techniques disclosed in Japanese Patent Application Post-Exam Publication No. Hei 6-46331 and Japanese Gazette Containing the Patent No. 2705090. In this case also, it is extremely difficult to impart the above-described three functions to the developer carrier even more surely.
Moreover, all the developing devices stated in the above-described official gazettes are of the non-contact development type. Therefore, the techniques disclosed in these official gazettes cannot be applied directly to contact development type developing devices in which the developer carrier contacts the latent image carrier.
Accordingly, an object of the present invention is to prevent the adhesion of a developer to a non-image area and to prevent the occurrence of fogging and blur due to disconnection, thickening or scattering of thin lines of the image.
Another object of the present invention is to eliminate the delay in the supply of a developer from a developer supply member and to allow the developer to be supplied stably even when an AC-superimposed bias voltage is applied to a developer carrier.
Still another object of the present invention is to provide a contact development type developing device that has a developer carrier capable of exhibiting three functions, i.e. developer conveying function, developer charging function, and developing bias voltage discharge preventing function, even more surely, and that allows the developer carrier to be formed simply at reduced costs.
To attain the above-described objects, the present invention provides a developing device including a developer carrier for carrying a developer. A supply member is disposed to rotate in contact with the developer carrier to supply a developer layer having a predetermined thickness to the surface of the developer carrier. A layer forming member is disposed to abut against the developer carrier to regulate the layer thickness of the developer so as to form a thin developer layer on the developer carrier. A bias application unit applies an AC-superimposed bias voltage to the developer carrier. The AC-superimposed bias voltage is formed by superimposing an alternating current on a DC bias voltage. A latent image on a latent image carrier is developed with the thin developer layer formed on the developer carrier by the layer forming member. The bias application unit sets the maximum value of the AC-superimposed bias voltage lower than the charge potential of the latent image carrier.
Preferably, the bias application unit sets the DC bias voltage lower than a middle potential between the charge and exposure potentials of the latent image carrier. The minimum value of the AC-superimposed bias voltage may be set lower than the exposure potential of the latent image carrier. The maximum and minimum values of the AC-superimposed bias voltage may be set so as to be identical in polarity with each other.
In addition, the present invention provides a developing device including a developer carrier for carrying a developer. A supply member is disposed to rotate in contact with the developer carrier to supply a developer layer having a predetermined thickness to the surface of the developer carrier. A layer forming member is disposed to abut against the developer carrier to regulate the layer thickness of the developer so as to form a thin developer layer on the developer carrier. A bias application unit applies an AC-superimposed bias voltage to the developer carrier. The AC-superimposed bias voltage is formed by superimposing an alternating current on a DC bias voltage. A latent image on a latent image carrier is developed with the thin developer layer formed on the developer carrier by the layer forming member. The bias application unit sets the minimum value of the AC-superimposed bias voltage higher than the exposure potential of the latent image carrier.
Preferably, the bias application unit sets the maximum and minimum values of the AC-superimposed bias voltage identical in polarity with each other. The maximum value of the AC-superimposed bias voltage may be set lower than the charge potential of the latent image carrier. The maximum value of the AC-superimposed bias voltage may be set higher than the charge potential of the latent image carrier.
In addition, the present invention provides a developing device including a developer carrier for carrying a developer. A supply member is disposed to rotate in contact with the developer carrier to supply a developer layer having a predetermined thickness to the surface of the developer carrier. A layer forming member is disposed to abut against the developer carrier to regulate the layer thickness of the developer so as to form a thin developer layer on the developer carrier. A bias application unit applies an AC-superimposed bias voltage to the developer carrier. The AC-superimposed bias voltage is formed by superimposing an alternating current on a DC bias voltage. A latent image on a latent image carrier is developed with the thin developer layer formed on the developer carrier by the layer forming member. The charge potential V0 and exposure potential Von of the latent image carrier, the peak-to-peak voltage Vpp of the AC-superimposed bias voltage and the DC bias voltage Vdc are set so as to satisfy the following conditions:
|V0xe2x88x92Von|xe2x89xa7|Vpp|
|Vdc|xe2x89xa6|V0xe2x88x92Von|/2
In addition, the present invention provides a developing device including a developer carrier for carrying a developer. A supply member is disposed to rotate in contact with the developer carrier to supply a developer layer having a predetermined thickness to the surface of the developer carrier. A layer forming member is disposed to abut against the developer carrier to regulate the layer thickness of the developer so as to form a thin developer layer on the developer carrier. A bias application unit applies an AC-superimposed bias voltage to the developer carrier. The AC-superimposed bias voltage is formed by superimposing an alternating current on a DC bias voltage. A latent image on a latent image carrier is developed with the thin developer layer formed on the developer carrier by the layer forming member. The bias application unit has a constant-current bias source for applying a constant-current bias voltage to the supply member to supply a constant current between the supply member and the developer carrier in such a manner as to follow the AC-superimposed bias voltage.
Preferably, the bias application unit includes an AC-superimposed bias source for applying the AC-superimposed bias voltage to the developer carrier and a constant-current bias source for applying the constant-current bias voltage to the supply member. The constant-current bias source has sufficiently high responsivity to follow the AC-superimposed bias voltage. The constant-current bias source is connected directly between the developer carrier and the supply member. The constant-current bias source follows the AC-superimposed bias voltage with a peak-to-peak voltage at least 0.5 times the peak-to-peak voltage of the AC-superimposed bias voltage.
In addition, the present invention provides a contact development type developing device having a developer carrier disposed in contact with a latent image carrier. The developer carrier carries a developer on the surface thereof to convey it to the latent image carrier. The developer carrier is formed from a metal roller. At least a developer carrier region of the surface of the metal roller is subjected to sandblasting treatment to form a dimpled surface. Further, at least the dimpled surface of the metal roller is subjected to aluminum anodizing treatment.
The developing device has a bias application unit for applying a developing bias voltage to the developer carrier. The developing bias voltage is an AC-superimposed bias voltage formed by superimposing an alternating current on a direct current. The developing bias potential is set closer to the electric potential set for the image area on the latent image carrier than the electric potential set for the non-image area on the latent image carrier. In other words, the developing bias potential is not set on the side of the non-image area electric potential remote from the image area electric potential. The circumferential speed of the developer carrier is set higher than the circumferential speed of the latent image carrier. The developer is a non-magnetic one-component toner prepared by externally adding an external additive having a predetermined hardness to toner particles. The hardness of the surface of the metal roller is set lower than the hardness of the external additive. The sphericity of the particles of the developer is set in the range of 0.9 to 1 in terms of Wadell""s practical sphericity.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.